For example, a differential protection relay samples and digitizes, at the same time, a current input by a current transformer at each terminal of a power line. The digitized current data is then mutually transmitted via a communication system, and each terminal performs a differential protection calculation by using its current and current data received from the opposite terminal to make a fault determination. Here, simultaneous sampling of current data at the same time is called sampling synchronization. A master terminal and slave terminals are determined among the terminals of the power line, and data regarding the sampling timing of each terminal is sent as a timing flag. Each terminal measures a time interval between its sampling timing and a reception time of the timing flag from the partner terminal. If the time interval is sent to the slave terminal from the master terminal, the slave terminal finds a sampling synchronization band. The sampling timing is corrected by the corresponding time. As a result, the sampling timings of the master terminal and the slave terminal can be synchronized. Such a synchronization control system is used not only in the differential protection relay but also in many transmission systems.
There has been known a conventional synchronization control system which is improved in precision by the correction of the time synchronized by a transmission control latency required when a synchronization signal is sent to a device having a clock therein. It is also known to make the precision of time synchronization between a time setter device and a device targeted for time setting within an allowable band even if a transmission delay time changes upon each transmission. Moreover, it is known that in a network system which comprises a serial transmission path which connects one master terminal and a plurality of remote stations, an overhead of sampling data processing is absorbed so that collected data is rapidly processed.